Carrier telegraph receiver



Dec. 29, 1936. "C. A. BORGESON 2,065,640

CARRIER TELEGRAPH RECEIVER Filed March 6, 1936 A TTORNEV Patented Dec. 29, 1936 UNITED `STATES PATENT oFFlcs 2,065.64@ Osama mnoasrn navman Carl Anders Bergeron. East Orange, N. J., as-

signor to American Telephone and Telegraph Company. a corporation of New York Application March 6, 1936, Sel'lll No. 67.549

- B Claims. (C1. ITS-88) This invention relates to carrier telegraphy or the like and relates more particularly to apparatus used on a channel compensated against level changes. The invention involves means for s preventing pulsing oi' a receiving relay and preventing lock-up arising dm'ing a spacing condition from various operating changes.

Transmission in telegraphy or in carrier telegraphy consists in the transmission of signal 1o pulses which, as they leave 'the transmitter. are substantially square-topped. The sidesv oi the waves after transmission over the line are sloped. Receiving apparatus is adjusted so that the time interval between transitions at the output oi! the receiving relay is substantially equal to the time interval between the same transitionain the original signal. 'I'he amplitudes of the signals as actually received do not remain equal. The signal amplitudes change from time to time giving zo rise to distortion known as telegraph bias. 'l'hese changes in received signal amplitudes may arise from changes in battery voltages, changes in line equivalent and from numerous other sources.

The problem oi' avoiding telegraph bias' has Ag5 been attacked in a number of ways. So far the method disclosed in Herman patent 1,886,808 is one of the most satisfactory methods of compepsation for telegraph bias and is in wide use. Compensation for bias at a receiver is provided according to the Herman patent, by the use o i a compensator relay controled from the receiver. relay and adapted to control the bias-of a receiving detectonto equalize the length of mark-l ing and spacing intervals at the receiving relay regardless of the changes in amplitudeof received pulses.

The present invention is in the nature oi an improvement upon the compensating circuit disclosed by Herman and is directed particularly to avoiding pulsing and lock-up oi.' the receiving relay under extreme service conditions.

An object of the present invention is to secure an increased reliability and to improve the operation o! a receiver in a carrier telegraph system or the like.

A further object ofthe invention is to provide means for avoiding pulsing in a carrier telegraph receiving system.

A still further object oi the invention is to 5o provide means for avoiding lock-upl in a carrier telegraph receiving system.

A feature ofthe invention' includes the provision of a resistance leak o! high value con'- necting the spacing contact oi a compensating relay to the grid side of the compensator condenser and arranged to shunt the compensator condenser which is employed for variably biasing the grid 'of a receiving detector to avoid telegraph bias.

In the drawing showing a preferred embodi- 5 mentof this invention:

Figure 1 isacircuitdiagramoiacarriertelegraph channel; v

Fig. 2 is a` circuit schematic for consideration of the theory relative to the present invention; 10 and v Fig. 3 is a characteristic curve ot receiver operation in a carrier telegraph receiving system.

Fig. 1 shows a carrier telegraph receiving system illustrating in particular a single channel 15 with connecting channels indicated. The system illustrated is substantially that of the Herman patent, noted above. except for a in the compensating means. e

Reference may be made to the Herman pat- 20 ent for a detailed discussion of the level compensator and its mode of operation. Only so much of the description will be given here as is helpful to anl understanding of the present invention. g5 Itwillbeunderstoodthatasthekey ii at the sending station is operated a succession of impulses of carrier waves from the source Il is sent over the line through the channel indicated on the drawing and these impulses are impressed 3 on the detector tube II at the receiving station, causing the polar relay 20 to operate, which in turn opens and closes the subscribers loop 21 shown leading to sounder 28.

Compensator relay 3l operates in unison with the polar relay 28 and during each marking interval connects grid leak resistance R in shunt'to the condenser C1 which is connectedin the grid circuit of the detector 2B. Normally the received carrier pulses drive the grid of the'jdetector 2U positive notwithstanding the 25-v'olt negative grid bias from the battery Il. During the positive pulses current is drawn througlitheresistance R throwing the grid biasm'ore negative in pro portion to the current ow through During the spacing interval this. increment 'of negative grid bias is held on the grid ldue to the chargev in the condenser. C1. A- received current stronger than normal increases still further the negative bias on the grid while a received current weaker than normal results in a smaller than normal negative grid bias, the overall result beingan equalizing eilect `in grid bias to compensate changes-inline equivalent. u

In order to understand how the invention prevents the occurrence of pulsing. it is first necessary to understand how pulsing occurs in a system in accordance with the Herman disclosure. It is known to begin with. thatif the sending key in the vHerman system be left open for an abnormally long period, forexample seconds, the receiving relay 2 0 is liable to being pulsing substantially at the dotfrequency. l

In an endeavor to get at the cause of this pulsing phenomenon-applicant observed from actual measurements in a large number of cases that (referring to Fig. 2) the leakage resistance R1 of the condenser C1 is so high as to be practically innnite; and further that the insulation resistance to ground, Rz, of the plate of the condenser C1 that is connected to the grid, while of high value '(for example 84 megohms) is not nearly as high as the resistance R1. It seems obvious that if both R1 and R2 were infinite, pulsing could not occur since the condenser C1 would hold a charge indefinitely and there would be no 4more reason for pulsing to occur after a 15-second interval than immediately upon opening the line. It was found impractical, however, to raise the value of Rz to a suiciently high value to prevent pulsing from occurring after an abnormally long line interruption.

Applicant's analysis employing the circuit diagram of Fig. 2 shows the following action to account for the occurrence of pulsing. It was known from the tube and relay characteristics (Fig. 3) that if the grid could always be kept 'at minus 25 volts thereV would be insufficient space current in the detector tube 2B to operate the relay since the minimum current for operating the relay 2i corresponds to agrid voltage of minus K, considerably smaller in absolute magnitude than minus 25 volts. Pulsing could not occur merely from the condenser C1 losing its charge for if this kwere the only change the grid potential would still be at minus 25 volts. What actually happens is that during a long spacing interval (Ra of Fig. 2 being considered absent), a small leakage current iiow takes place from battery 32 through the resistances R1 and Rz in series in a direction to neutralize any negative charge on the condenser C1 and to build up a charge in the opposite direction. The resistances R1 and Rz act like a potentiometer and the potential which the grid ilnally reaches will depend upon the ratio of these two resistances. Since in the actual cases under observation R1 was much larger than R1 the grid was brcughtto a potential of only a few volts negative, suillciently low` negative to permit relay 2B to operate, this in turn operating relay 30 which by connecting R across condenser C1 discharged the' condenser and partially restored the bias on tube 25 toward its normal value of 25 volts negative. In this process the relay 2l would be released and the process would repeat itself, continuing the pulsing indefinitely.

Applicant discovered a simple and inexpensive cure of this pulsing evil in the form of a resistance R1 which may be connected either between the plates of condenser C1 or in the preferred form, as shown, may be connected from the grid side of condenser C1 to the spacing contact of the wmpensating relay 30. 'I'he value of this resistance is preferably made slightly more than half that of the resistance R1, or in the example under consideration, 4'7 megohms. With this resistance, when a long open line period occurs, a circuit exists from battery $2 to the spacing contact of relay 3l, resistance R1 and resistance R1 in series permitting a leakage current to flow in a direction to neutralize the normal negative charge on condenser C1 and to build up in this case a small positive charge of a value determined by the relative magnitudes of R1 and R1. Using resistances of the values indicated, i. e.. R1 equal to 84 megohms and Rs equal to 47 megohms, the maxi; mum change permitted in a positive direction of the voltage of the grid of detector tube 2l will be suilicient only to bring the grid voltage to a value of approximately 17 volts negative, regardless of the length of time the line interruption may continue. This approximate value is represented on the curve of Fig. 3 by Q and, as shown.

is suiiiciently removed in a negative direction from the relay operate value K to insure that the relay will remain unoperated.

Lock-up occurs when the incoming signals areI too weak to overcome the negative bias of the detector grid to a suiiicient extent to operate the relay 26. As noted above, strong incoming signals develop a relatively high negative bias for the grid of tube 2l by grid rectifying action and the resultant current iiow through the resistance R.. Although the voltage oi' the grid battery 32 may be only 25 volts, it isv not unusual for the grid bias to amount to 125 volts negative, the extra volts being caused by grid current flow. If .there is now a sudden reduction in received level, for example, 6. decibels, and if a relatively high negative charge has been accumulated on the condenser C1 and is allowed to remain there.

.it may happen that the incoming signals do not carry the grid swing sumciently to the right in Fig. 3 to reach the value K required to operate the relay 28. Instances have been known where portions of a message sent over a line under these conditions have been lost because of failure of the receiving relay to operate. Such conditions arise, for example, from a sudden shift in line equivaient, or more often from the interchange of circuits on. which widely differing levels have existed. Attendants have avoided this difiiculty in the past by momentarily shorting the condenser C1 in each of the receiving channels when lines are interchanged. ,1

'Ihe provision of the resistance Re. as explained above, has not only completely solved the pulsing dliiiculty as described, but also prevents the lockup of the receiving relays by providing a discharge path for the condenser C1. With a value for this resistance as indicated. the discharge time of the condenser C1 is made suillciently slow to permitA eifective operation of the level compensating action, and yet is suiliciently rapid to prevent lock-up either from sudden reduction in receivt'sd signal level or from the interchange of circui While, as noted above, the resistance R1 could .be directly connected across the plates of the condenser C1, it is preferred to connect it to the spacing contact of the compensator relay so as not contactofsaidrelaysuchthatitisconnectedin 7l shunt of said condenser during intervals of no received signal energy.

2. The combination according to claim 1 in which the value of said other resistance is proportioned with respect to the insulation resistance to ground of the circuit on the side of said condenser tovvard the grid, to x the eventual grid potential produced by leakage current flow after a long non-signaling interval at a negative value numerically greater than the value corresponding to the operate point of the relay.

3. The combination according to claim 1 in which the values of said condenser and firstmcntioned resistance are high enough to give eicctive compensation for the variable strength of received signals and in which the value of the second resistance is made high enough not to interfere with such effective compensation but suiiciently low to provide with the condenser a time constant small in comparison with the times in which large changes in line attenuation occur, whereby lock-up of the circuit is prevented.

4. In a receiver in an electrical telegraph system, a vacuum tube having three elements, a compensating relay with marking and spacing contacts, a bias battery connected to the grid of the tube, a network in the biasing circuit of the tube, said network including a condenser connected to the relay armature and a resistance connecting the marking Contact of the compensating relay to the grid and in shunt of the condenser, and means connecting the battery to the grid through the spacing contact of the compensating relay to cause the grid bias to remain suiiciently negative not to reach the receiver operating point during a spacing interval.

5. Inv a receiver station in an electrical telegraph system, a compensating relay with marking and spacing contacts, a vacuum tube having a grid element, a battery connected over the armature and marking contact of the compensating relay to the grid of the tube to vary the bias of the grid with variations in the level of the received signals, a network between the battery and the grid of the detector tube including a condenser and a potentiometer divider, the potentiometer divider including a resistance connecting the marking contact of the compensating relay to the grid in shunt of the condenser, and a resistance on the battery in series with the condenser and in series with said armature and a relatively high resistance connecting the spacing contact of the compensating relay to the grid to maintain the grid suiliciently negative to prevent the detector passing a pulse of current during a spacing interval.

6. In a receiver for a carrier telegraph system, a detector vacuum tube, a battery to render the grid of the tube negative, a compensating relay having marking and spacing contacts, a network in the grid circuit of the detector tube cooperating with the relay to avoid telegraph bias,r 

